The present invention relates in general to demodulation circuits and, more particularly, to a demodulation circuit that operates over a wide range of modulation frequencies.
Modern communication systems generally use modulation techniques to transmit signals over a medium such as airways or communication lines. Typical applications include cordless and cellular telephones, pagers, and other radio equipment. For example, a data communication signal is amplitude and phase modulated for transmission to a receiving site. The received data communication signal must be demodulated for end use. A variety of modulation techniques are known in the art including quadrature phase shift keying and quadrature amplitude modulation. Both of these modulation schemes combine the well known amplitude modulation and phase or frequency modulation as is used in FM radio. The quadrature modulation schemes have been widely adopted for their efficiency in transmitting information over the communication link.
On the receiving side it is necessary to demodulate the modulated communication signals. The demodulation process typically involves generating an in-phase replica of the modulated suppressed carrier and a quadrature replica of the suppressed carrier that is 90 degrees out of phase with respect to the in-phase carrier signal. In the prior art, the received modulated signal is multiplied by the in-phase and quadrature components of the recovered carrier signal to recover the baseband data communication signal. The incoming modulated signal is typically mixed down from a carrier frequency in the gigahertz (GHz) range down to an intermediate frequency of say 450 megahertz (MHz). The IF signal is filtered by a surface acoustic wave filter and amplified by an IF amplifier. The resulting IF signal is then multiplied by the in-phase and quadrature components of the recovered carrier to provide in-phase (I) and quadrature (Q) baseband signals.
A common technique of generating the in-phase and quadrature recovered carrier signals is to generate a frequency at four times the IF frequency, for example with a phase lock loop, and then divide down that frequency in two steps of division by two each step to provide the in-phase and quadrature components. It can be difficult to generate a stable low phase noise oscillator operating at four times the IF carrier frequency because the inductor and capacitor associated with such an oscillator become impractical at very high frequencies.
Another problem with the prior art demodulation is the inflexibility in operating at different IF carrier frequencies. Depending on the country of use the standard IF carrier may range from 40 MHz to 450 MHz. The oscillator and demodulation circuit is designed to a particular IF frequency of operation. For those systems needing to operate at more than one IF carrier frequency separate oscillators are needed each tuned to the predetermined carrier frequency of operation.
Hence, a need exists for a demodulation circuit that oscillates with a lower frequency oscillator while providing flexibility and operating with a variety of IF carrier frequencies.